For example, Patent Literature 1 discloses a catalyst device for reducing and removing NOx. The catalyst device of Patent Literature 1 reduces and removes NOx with HC in a fuel as a reducer. The catalyst device has two catalysts with different cell sizes in an inside thereof. The two catalysts are disposed in series side by side so that the catalyst with a larger cell size is located at an upstream side.
Further, there has been conventionally known an exhaust cleaning system in which a three-way catalyst (TWC), an NOx storage reduction catalyst (NSR catalyst) and an NOx selective catalytic reduction catalyst (SCR catalyst) are disposed in this order from the upstream side of the exhaust passage of an internal combustion engine. Bank control or rich spike is executed, whereby ammonia (NH3) is generated in the three-way catalyst and the NSR catalyst, and the generated ammonia (NH3) is supplied to the SCR catalyst. In the SCR catalyst, the supplied ammonia (NH3) is used, and the NOx that flows therein is selectively reduced. The system like this aims to enhance the removal performance for NOx by further removing the NOx that remains without being removed by the three-way catalyst and the NSR catalyst, in the SCR catalyst.